#include <bits/stdc++.h>
using namespace std;

int main() {
	double n;
	cin >> n;
	double dp[3] = {1, 1, 0};
	for (int i = 2; i < n; ++i) {
		dp[2] = dp[0] + dp[1];
		dp[0] = dp[1];
		dp[ 1] = dp[2];
	}
	printf("%.2lf", dp[2]);
	return 0;
}